Current state-of-the art microelectromechanical (MEMs) resonators have recently challenged the quartz-dominated timing industry, notwithstanding the perceived temperature instabilities of such MEMs devices, which has led to the development of very high performance temperature sensors in MEMs-based circuits to thereby support levels of frequency stability that are typically associated with crystal oscillators (XO). Moreover, because one of the significant advantages of MEMs-based technology is the highly integrated semiconductor manufacturing technologies that support high volume wafer-level encapsulation of MEMs components, efforts have been made to determine how crystal-based resonators can benefit from similar wafer-level scale and vacuum sealing packaging technologies. One example of a wafer-scale packaging technology is disclosed in U.S. Pat. No. 7,955,885 to Bhugra et al., entitled “Methods of Forming Packaged Micro-Electromechanical Devices,” the disclosure of which is hereby incorporated herein by reference.
An example of a conventional real time clock (RTC) module includes a hybrid combination of an application specific integrated circuit (ASIC) and a crystal blank inside a ceramic package. However, as disclosed in an article by Ruffieux et al., entitled “A Versatile Timing Microsystem Based on Wafer-Level Packaged XTAL/BAW Resonators with Sub-uW RTC Mode and Programmable HF Clocks,” 2013 IEEE International Solid-State Circuits Conference, Feb. 19, 2013, pp. 196-198, a ceramic package may be replaced with silicon and the circuit may be implemented as an active part of the package. U.S. Pat. No. 8,035,454 to Navet discloses a crystal oscillator device containing a piezoelectric resonator element and integrated circuit (IC) chip within a vacuum chamber.
Quartz, MEMs and LC-based timing applications can utilize inductors to support resonance frequencies and/or timing circuit operation (e.g., VCO operation). However, achieving high-quality (i.e., high-Q) inductors can often be difficult using conventional wafer-scale fabrication techniques. As will be understood by those skilled in the art, high-Q inductors require low series resistance, smooth metal surfaces and small grain structure. Typical inductor materials include copper and silver, but not aluminum. Moreover, because inductors create surrounding fields, forming inductors adjacent lossy materials will significantly reduce their Q. Accordingly, high value inductors, such as spiral inductors, may provided insufficiently high Q when formed on an integrated circuit (IC) substrate having relatively low resistivity. One example of a spiral inductor is disclosed in commonly assigned U.S. Pat. No. 6,798,039 to Gillespie et al., entitled “Integrated Circuit Inductors Having High Quality Factors,” the disclosure of which is hereby incorporated herein by reference. Another example of an inductor that utilizes encapsulating shielding layers to inhibit a loss in Q is disclosed in commonly assigned U.S. Pat. No. 8,164,159 to Armstrong et al., entitled “Semiconductor Resonators with Electromagnetic and Environmental Shielding and Methods of Forming Same,” the disclosure of which is hereby incorporated herein by reference.
Efforts have also been made to manufacture packaged crystal-based oscillator circuits having reduced sensitivity to acceleration and vibration forces. One such effort is disclosed in U.S. Pat. No. 4,575,690 to Walls et al., entitled “Acceleration Insensitive Oscillator,” which discloses a crystal-based oscillator circuit including two crystals of unequal acceleration sensitivity magnitude and mounted such that their respective acceleration sensitivity vectors are aligned in an anti-parallel relationship. Similar efforts are also disclosed in U.S. Pat. No. 7,915,965 to Hardy et al., entitled “Oscillator Having Reduced Sensitivity to Acceleration,” and in U.S. Pat. Nos. 8,188,800, 8,525,607 and 9,054,635 to Fry et al., entitled “Crystal Oscillator with Reduced Acceleration Sensitivity.”